1. Field of the Invention
The present invention relates to b method of electrically contacting a semiconductor surface coated with at least one dielectric layer, which surface is to be electrically contacted, particularly for contacting a p-conductive basic layer of a solar cell coated with a dielectric passivation layer.
2. Description of the Prior Art
For reasons of competition already, the industrial manufacture of solar cells is governed by the approaches to produce solar cells with a maximum efficiency, that is with an electric current yield as high as possible from the energy flow arriving on the solar cell and to keep the manufacturing expenditure together with the manufacturing costs, which are closely linked up therewith, as low as possible.
For a better understanding of the measures to be taken into consideration for an optimized manufacture of solar cells, the following statements are presented.
Solar cells are devices converting light into electric energy. Usually they consist of a semiconductor material—solar cells are mostly manufactured from silicon—that has n-doped or p-doped semiconductor regions. The semiconductor regions are referred to as an emitter or as a base, respectively, in a manner known per se. The light incident on the solar cell serves to generate positive and negative charge carriers within the solar cell, which are separated in space from each other at the interface between the n-doped (emitter) and the p-doped (base) semiconductor regions, at the so-called pn-junction. Metal contacts may be used, which are connected to the emitter and the base, to carry off these charge carriers separated from each other.
In the simplest form, solar cells consist of base regions 2 and emitter regions 3 over the entire area, with the emitter 3 being located on the side that faces the light, that is the face side of the solar cell. For illustration, reference should be made here to FIG. 1 that shows a known solar cell.
For establishing the electric contact of the base 2, usually the rear side of the solar cell 1 is provided with a metal layer 4 over its entire area, onto which appropriate rear-side contact conductors 5, e.g. of AlAg, are applied. The emitter region 3 is contacted with a metal grid 6 with the objective being to lose as little light as possible by reflection on the metal contact for the solar cell, that is the metal grid 6 presents a finger structure so as to cover a minimum of solar cell surface. For an optimization of the power yield of the solar cell 1, an attempt is made to keep the optical losses caused by reflection at a minimum. This is achieved by the deposition of so-called anti-reflection coatings 7 (ARC) on the surface of the face side of the solar cell 1. The thickness of the layer of the anti-refection coatings 7 is so selected that in the energetically most important spectral range a destructive interference of the reflective light occurs. Anti-reflection materials used may be titanium dioxide, silicon nitride and silicon dioxide. As an alternative or additionally, it is possible to achieve a reduction of reflection by the production of a suitable surface texture by means of an etching or mechanical processing method, as is also evident from the solar cell illustrated in FIG. 2. Here, the emitter region as well as the anti-reflection layer 7 applied on the emitter are configured to present such a structure that the light incident on the structured surface of the solar cell displays an increased probability of coupling on the structures designed to have a pyramid-like shape. In the case of the solar cell according to FIG. 2, too, electric contact of the emitter 3 is established with a metal grid 6 being as fine as possible, whereof only a narrow contact finger is shown in FIG. 2. The anti-reflection coating 7 on the face side may also serve as passivation layer that ensures, on the one hand, a mechanical protection of the surface and, on the other hand, presents also intrinsic effects with respect to the reduction of superficial recombination processes that will be discussed in more details below.
When a solar cell is electrically contacted, it is necessary to distinguish between the face side and the rear side. While on the rear side of the solar cell the establishment of a contact is attempted, which provides a low contact and conducting resistance, it is additionally necessary on the face side to couple a maximum of light into the solar cell. For this reason, normally a comb structure is created on the face side, as is obvious from FIG. 1, in order to keep both the resistance losses and the shading losses small. Usually, contacts are used which cover the entire area or which are structured such as in a grid-like manner.
The surfaces of solar cells of high efficiency excel themselves not only by good electric contacts but additionally also by a low rate of superficial recombination, which means that the probability of minority charge carriers arriving on the surface of the solar cell and possibly recombining there so that they do not contribute to the production of energy is low, which would result in a substantial reduction of the efficiency.
This can be realized either by the effects that (a) only a few minority charge carriers arrive on the surface, or that (b) they recombine on the surface with a low probability only.
The method (a) can be realized by the provision that a high level of doping with foreign atoms is created in the region of the surface or that a dielectric layer is applied on the surface while invariable charges are integrated into the interface plane between the semiconductor and the dielectric layer. A high doping level is realized by doping the emitter on the face side with different levels whilst, as a supporting provision, a rear side field—a so-called “back surface field” may be integrated on the rear side.
A high doping level is however, always has the disadvantage that even though the probability of recombination can be reduced on the surfaces of the solar cell, the probability of recombination within the solar cell layer is increased. Charges may also be integrated, for instance, by a silicon nitride layer that serves particularly well as anti-reflection coating.
The method (b) can be realized by the provision that the superficial recombination states are reduced, for example, by saturation of silicon bonds that are broken up on the surface and are hence not completely saturated by a layer of silicon nitride or silicon dioxide, which, as described above, may be used on the face side also as anti-reflection coating. This passivation may be applied on both the face side and the rear side and constitutes one of the most important features of highly efficient solar cells.
Another feature of such highly efficient solar cells are narrow (<40 pm) and high face side contacts (>10 mm) presenting a low contact and conduction resistance. The surface contacts configured as grid fingers should cover the smallest solar cell area possible, which means that they must be configured in the narrowest form possible. Moreover, the grid fingers should present the lowest conduction resistance possible for carrying off the charge carriers separated in the solar cell, which means that their cross-section should be as large as possible.
The most important known techniques of metallization for the rear side contacts of a solar cell are as follows:
(A) Screen Printing Technique
    An aluminium paste is printed through a screen over the entire area on the surface. Then, in a high-temperature step, a temperature of roughly 700 to 800° C. is maintained for roughly 10 to 30 seconds. This operation realizes a sound electrical contact whilst an aluminium silicon alloy—the “back surface field”—is formed. This is the process most commonly applied in industry for producing the rear side contacts.(B) Vapour Deposition Over the Entire Area    The metal coating is applied by vapour deposition over the entire area.(C) Photolithography and Vapour Deposition    First of all, a mostly passivating dielectric layer such as silicon dioxide is applied. The desired structure is then exposed down to the previously applied dielectric layer by exposure, development and washing of a photosensitive film, the so-called etching resist. This dielectric layer is then opened up to the silicon wafer in a subsequent etching operation. On the rear side of the solar cell, metallization is possible immediately after the layer has been opened and the photosensitive etching resist has been removed. Then the rear side contact may be applied over the entire area, for example by vapour deposition.
The known photolithographic process is suitable for the production of structure sizes down to less then 1 μm. Photolithography, however, is a comparatively expensive technique and is therefore not usually applied in the industrial field of solar cell manufacture. The majority of processes applied so far for the production of solar cells having an efficiency better than 20% include several photolithographic steps of process. The solar cell described above with reference to FIG. 2 can be produced with application of the two aforedescribed photolithographic steps.
(D) Photolithography, local high doping and vapour deposition One variant of this method is the application of a locally high doping level among the contacts, which improves, above all, the contact characteristics. The known realization of local high doping is achieved by diffusing a doping substance that is to be applied prior to diffusion and that must possibly be removed again subsequently. Finally, the contacts are established as described above in item (C).
With the application of these methods, the highest efficiency levels of roughly 24% have been realized, which have so far been achieved on silicon. However, the process sequence is extremely expensive and complex and is therefore left out of consideration for the manufacture of solar cells on an industrial scale.
(E) A method suitable for producing the face side contact of a solar cell in a manner partly resembling photolithography has been described in the U.S. Pat. No. 5,011,565 “Dotted Contact Solar Cell and Method of Making Same” by Dube et al. The patent describes a solar cell type and a method of its manufacture. The face side of the solar cell is provided there with a dielectric coating that is opened on dots disposed in lines by means of a laser, specifically a YAG laser. The dots are applied at a defined spacing. The formation of the contacts proper is then realized by the deposition of nickel and copper in a chemical bath. This operation bridges the distances between the dot contacts.(F) A similar contacting method is described in the U.S. Pat. No. 4,626,613. There, troughs are formed through a passivating dielectric layer in the surface of the solar cell for contacting, which troughs are subsequently filled with a contact metal. The trough is created by mechanical structuring techniques or preferably by laser ablation. The method is used for contacting the cell face side on an industrial scale.(G) The patent document PCT/AU99/00871 discloses a contacting method based on diffusion from a semiconductor layer, wherein a metal layer is contacted through an electrically insulating intermediate layer and a semiconductor layer of a low doping level with a second electrically conductive semiconductor layer presenting a high doping level, by means of the application of energy at an optional level, using a laser source. Due to the high light energy introduced, diffusion from the semiconductor layer commences, which is simple to contact in view of its high doping level and which can be accepted for the formation of the electrical contact in the sense of the embodiments described in that prior art reference. When this technique is applied, a semiconductor layer is contacted that is simple to contact in terms of process engineering, on account of its high doping level, however, the electrically insulating separating layer as well as an intermediate semiconductor layer that is not to be contacted suffers substantial damage at an unacceptable level, which is caused by the high energy supply required.(H) A further similar method for contacting the rear side is known from the publication of “R. Preu, S. W. Glunz, S. Schaefer, R. Luedemann, W. Wettling, W. Pfieging, entitled “Laser Ablation —A New Low-Cost Approach for Passivated Rear Contact Formation in Crystalline Silicon Solar Cell Technology”, Proceedings of the 16th European Photovoltaic Solar Energy Conference, Glasgow, UK 2000. There, the rear side of a solar cell is contacted by applying first a passivating dielectric coating over the entire area, which coating is subsequently locally opened by means of a laser with short pulses. Then, an aluminium layer is applied over the entire area. A sounds electrical contact is created by heating the wafer up to 400° C. or more.(I) Furthermore, a method has become known wherein a dielectric coating is applied over the entire area and subsequently a paste is locally applied—for instance by means of a screen printing technique—which contains, inter alia, also etching components in addition to metal components. When the temperature is increased this etching process is launched or accelerated so that the dielectric coating is locally opened and that a sound electric contact may be formed between the metal-containing paste and the substrate.
The solar cells manufactured to date on an industrial scale with application of the technologies of screen printing of an aluminium paste over the entire area (A) as well as of vapour deposition over the entire area, which have been briefly outlined in the foregoing, present an efficiency that is definitely lower than the efficiency of solar cells manufactured by application of the photolithography technology. A higher efficiency ratio means, however, a distinctly increased value of the solar cell. The application of the technologies (C) and (D) based on the techniques of photolithography is, however, so expensive and complex at present that it is not realized, despite the high efficiency ratios achievable.
According to the methods described in the U.S. Pat. Nos. 5,011,565 and 4,626,613 the silicon present underneath is so substantially damaged during removal of the dielectric coating that in practical operation one part of the silicon material must be removed with an additional etching step.
Moreover, in laser ablation, frequently the problem arises that the material ablated by the laser beam deposits on the surface to be processed and deposits on optical imaging units such as collecting lenses, which are possibly present in the light beam path, which involves a substantial negative influence on the removal process, interruptions of the removal process are the consequence for permitting necessary cleaning work.
Similar problems relating to cleaning occur in the method (G): In that case, however, the damage created by fusion of the entire region disposed underneath the field of laser effect, specifically in the dielectric coating, renders this method unsuitable for the application of highly efficient solar cell concepts. Moreover, the known method is merely one possibility of contacting highly doped semiconductor layers.
Finally, in the laser ablation method (H), the contact requires subsequent processing at temperatures higher than 300° C. after application of the metal coating, in order to achieve very good resistance values, which means an additional step in the process that restricts moreover the selection of the dielectric passivation layers.
The local application of the metal-containing paste containing also etching substances in correspondence with method (I) involves the disadvantage that the production of the paste is complicated and expensive and hence causes distinctly higher costs than the use of a pure metal that may be used, for instance, in vapour deposition. Apart therefrom, the surface must be cleaned prior to metallization. Moreover, in the contact forming process, the entire area of the rear side is exposed to high temperatures, which restricts the selection of possible passivation materials or which may result in an impairment of the passivation layer. Furthermore, when the rear side is only locally metallized, for example, in the form of a grid, the reflectivity of the rear side is reduced for an increase of the light capture inside the cell, compared against metallization of the entire area. For this reason, the reflection of light of a wavelength absorbed only slightly in the photovoltaic active material is substantially worse than in the case of a rear side metallized over the entire area.